I would say this is a mod_rewrite and permalink issue. If it was working before, then something changed with the host, maybe the .htaccess file in the forum directory, or the parent one. But, all you need to do to get around it, for now, is turn off the permalinks, to false.

I think you need to set up permalinks on your server if you want pretty or name based ones. False works fine, so it’s definitely a permalinks issue. Not sure why it was working before, but this is what needs to be done now:

I am experiencing the same issues with my bbPress install. It was working fine before and I have not changed anything either. I figure the host (Dreamhost) has changed some configuration or other. I have contacted them and am waiting to hear back.

No word back from Dreamhost support, but it appears as if many DH customers are experiencing similar issues with other applications. I managed to fix my issue by logging into my DH account and changing the PHP version from 5.2.x -> 4.4.x and then back to 5.2.x.